Standard deep bed filters consist of carefully graded layers of granular filter media. Typically, such a filter bed may consist of one or more layers of anthracite superimposed upon lower layers of sand. Several attempts have been made to provide an uppermost layer of a larger, more porous lightweight material which can act as a buffer or strainer for removing larger particles, flocculents, or high concentrations of contaminants which have a tendency to surface load and plug the upper exposed surfaces of standard deep bed filters. Such particles for the upper surface should have the characteristics of, (1) water-wettability, (2) relatively large, uniform size, (3) relatively low, controlled density, and (4) liquid impermability to avoid density change during use.
The provision of such materials would avoid surface clogging or blinding of the filter bed, minimize intermingling of the layers of the filter bed after backwashing, and prevent either the adherence of air bubbles to the particles or the absorption of liquid by the particles which changes the density of the particles.